Single crystal group III nitride articles and method of producing same by HVPE method incorporating a polycrystalline layer for yield enhancement

ABSTRACT

In a method for making a GaN article, an epitaxial nitride layer is deposited on a single-crystal substrate. A 3D nucleation GaN layer is grown on the epitaxial nitride layer by HVPE under a substantially 3D growth mode. A GaN transitional layer is grown on the 3D nucleation layer by HVPE under a condition that changes the growth mode from the substantially 3D growth mode to a substantially 2D growth mode. A bulk GaN layer is grown on the transitional layer by HVPE under the substantially 2D growth mode. A polycrystalline GaN layer is grown on the bulk GaN layer to form a GaN/substrate bi-layer. The GaN/substrate bi-layer may be cooled from the growth temperature to an ambient temperature, wherein GaN material cracks laterally and separates from the substrate, forming a free-standing article.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/015,303, filed on Jan. 27, 2011, which is a continuation of U.S.patent application Ser. No. 11/606,783, filed on Nov. 30, 2006, issuedas U.S. Pat. No. 7,897,490, which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 60/749,728, filed Dec. 12, 2005, titled“Bulk Gallium Nitride Crystals and Method of Making the Same;” U.S.Provisional Patent Application Ser. No. 60/750,982, filed Dec. 16, 2005,titled “Method of Producing Freestanding Gallium Nitride bySelf-Separation;” U.S. Provisional Patent Application Ser. No.60/810,537, filed Jun. 2, 2006, titled “Low Defect GaN Films Useful forElectronic and Optoelectronic Devices and Method of Making the Same;”U.S. Provisional Patent Application Ser. No. 60/843,036, filed Sep. 8,2006, titled “Methods for Making Inclusion-Free Uniform Semi-InsulatingGallium Nitride Substrate;” and U.S. Provisional Patent Application Ser.No. 60/847,855, filed Sep. 28, 2006, titled “Method of Producing SingleCrystal Gallium Nitride Substrates by HVPE Method Incorporating aPolycrystalline Layer for Yield Enhancement,” the contents of which areincorporated by reference herein in their entireties.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant numberN00164-04-C-6066 by the Missile Defense Agency (“MDA”). The UnitedStates Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for producing gallium nitride (Al, Ga,In)N single crystal substrates that are useful for producingoptoelectronic devices (such as light emitting diodes (LEDs), laserdiodes (LDs) and photodetectors) and electronic devices (such as highelectron mobility transistors (HEMTs)) composed of III-V nitridecompounds.

2. Description of the Related Art

Group III-V nitride compounds such as aluminum nitride (AlN), galliumnitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN,and AlGaInN, are direct bandgap semiconductors with bandgap energyranging from about 0.6 eV for InN to about 6.2 eV for AlN. Thesematerials may be employed to produce light emitting devices such as LEDsand LDs in short wavelength in the green, blue and ultraviolet (UV)spectra. Blue and violet laser diodes may be used for reading data fromand writing data to high-density optical discs, such as those used byBlu-Ray and HD-DVD systems. By using proper color conversion withphosphors, blue and UV light emitting diodes may be made to emit whitelight, which may be used for energy efficient solid-state light sources.Alloys with higher bandgaps may be used for UV photodetectors that areinsensitive to solar radiation. The material properties of the III-Vnitride compounds are also suitable for fabrication of electronicdevices that may be operated at higher temperature, or higher power, andhigher frequency than conventional devices based on silicon (Si) orgallium arsenide (GaAs).

Most of the III-V nitride devices are grown on foreign substrates suchas sapphire (Al₂O₃) and silicon carbide (SiC) because of the lack ofavailable low-cost, high-quality, large-area native substrates such asGaN substrates. Blue LEDs are mostly grown on insulating sapphiresubstrates or conductive silicon carbide substrates. Sapphire belongs tothe trigonal symmetry group, while SiC belongs to the hexagonal symmetrygroup. GaN films and InGaN films have been heteroepitaxially grown onthe c-plane sapphire surface for LED devices. Due to lattice mismatch,the GaN films grown on both sapphire and SiC substrates typically havehigh crystal defects with a dislocation density of 10⁹ to 10¹⁰ cm⁻³.Despite the high defect density of the LEDs grown on these substrates,commercial blue LEDs produced from these materials have long lifetimessuitable for some applications.

UV LEDs based on alloys of GaN, however, show strong dependence of thepower output on the substrate material used. The UV LEDs can be grown onnative GaN substrate or on foreign substrates such as sapphire andsilicon carbide. On the foreign substrate, a GaN or AlGaN thin film isfirst grown by utilizing appropriate techniques and the active UV LEDstructure is subsequently grown. It has been found that the power outputof UV LEDs grown on native GaN substrate is much greater than the poweroutput of those grown foreign substrates (see, for example, Yasan et al.Applied Physics Letters, Volume 81, pages 2151-2154 (2002); Akita et al.Japanese Journal of Applied Physics, Volume 43, pages 8030-8031 (2004)).The lower density of crystal defect of the device structure grown onnative GaN substrate contributes to higher power output.

Group III-V nitride-based laser diodes also show a remarkable dependenceof lifetime on the crystal defect density. The lifetime of these LDsdramatically decreases with the increase of the dislocation density(see, for example, “Structural defects related issues of GaN-based laserdiodes,” S. Tomiya et al., MRS Symposium Proceedings, Vol. 831, p. 3-13,2005). Low-defect density single-crystal gallium nitride substrates areneeded for the long lifetime (>10,000 hours) nitride laser diodes.

Because of the very high equilibrium nitrogen pressure at the meltingpoint, gallium nitride single crystals cannot be grown with conventionalcrystal growth methods such as the Bridgman method or Czochralski methodwhere single crystals are grown from the stoichiometric melt. At ambientpressure, GaN starts to decompose well before melting.

Hydride vapor phase epitaxy (HVPE) has been utilized to grow relativelythick GaN on foreign substrates. In the HVPE process, gallium chloride(GaCl), formed by reacting gaseous hydrochloric acid (HCl) with galliummetal upstream in the reactor, is transported to the crystal growthregion where it reacts with ammonia, depositing GaN on the surface of asubstrate. The size of the GaN crystal grown may be the same size as thesubstrate. Substrates such as sapphire, gallium arsenide, siliconcarbide, and other suitable foreign substrates have been used.

Vaudo et al. in U.S. Pat. No. 6,440,823 discloses a method of producinglow defect GaN using HVPE on sapphire substrates. The sapphire substratecan be removed to produce a large area GaN substrate, for example, by alaser induced liftoff process as described by Kelly et al. (“Largefreestanding GaN substrates by hydride vapor phase epitaxy andlaser-induced liftoff,” Jpn J. Appl. Phys., Vol. 38, L217-L219, 1999).The wavelength of the laser beam, or the energy of the laser beam, ischosen so that it is smaller than the bandgap of the substrate, butlarger than the bandgap of GaN. The substrate is transparent to thelaser beam, but the GaN absorbs the laser energy, heating the interfaceand decomposing the GaN at the interface, which separates the GaN filmfrom the substrate. In U.S. Pat. App. Pub. No. 2002/0068201, Vaudo etal. further discloses a method of producing freestanding GaN near thegrowth temperature by shining a laser beam at the interface between thegrown GaN layer and the template, and decomposing the interfacematerial. This process involves dangerous high-energy laser beams andhigh manufacturing cost.

Chin Kyo Kim in U.S. Pat. No. 6,923,859 discloses an apparatus andassociated manufacturing method for GaN substrates in which a substrateand a GaN layer are separated from each other after growing the GaNlayer on the substrate in the same chamber. The apparatus contains atransparent window at the circumference of the chamber to allow theintroduction of the laser beam to the substrate. This process likewiseinvolves dangerous high-energy laser beams and has high manufacturingcost.

Bong-Cheol Kim in U.S. Pat. No. 6,750,121 discloses an apparatus andmethod for forming a single crystalline nitride substrate using hydridevapor phase epitaxy and a laser beam. After growth of the GaN film onsapphire substrate, the wafer is moved to a heated chamber forlaser-introduced separation. Because the wafer does not cool to roomtemperature, cracking induced by the mismatch of the coefficient ofthermal expansion is eliminated. This process likewise involvesdangerous high-energy laser beams and has high manufacturing cost.

Park et al. in U.S. Pat. No. 6,652,648 discloses a method of producingGaN substrate by first growing HVPE GaN on sapphire substrates. Thebackside of the sapphire substrate is protected for minimal parasiticdeposition. After GaN growth on sapphire substrate, the GaN/sapphirestructure is removed from the reactor. The GaN layer is subsequentlyseparated from the sapphire substrate by a laser liftoff process. Inaddition to involving dangerous high-energy laser beams, the GaN layeron sapphire is likely to crack upon cool down, and thus this processsuffers with low yield and high manufacturing cost.

Motoki et al. in U.S. Pat. No. 6,693,021 discloses a method of growingthick GaN film on gallium arsenide (GaAs) substrate. The GaAs substrateis wet-etched away to produce a free-standing GaN substrate. However,GaAs substrates tend to thermally decompose at the GaN growthtemperature and in the GaN crystal growth environment, introducingimpurities to the GaN film.

Yuri et al. in U.S. Pat. No. 6,274,518 discloses a method for producinga GaN substrate. A first semiconductor film (AlGaN) layer is formed on asapphire substrate, and a plurality of grooves is formed on the AlGaNlayer. A relatively thick GaN film is grown on a grooved AlGaN templateby an HVPE method, and upon cooling down from the growth temperature toroom temperature, GaN separates from the template, forming a large areafreestanding GaN substrate. However, this method requires deposition andpatterning of several films in different systems andcracking-separation. Thus, the process is one of low yield and highmanufacturing cost.

Solomon in U.S. Pat. No. 6,146,457 discloses a thermal mismatchcompensation method to produce a GaN substrate. The GaN film isdeposited at a growth temperature on a thermally mismatched foreignsubstrate to a thickness on the order of the substrate, where thesubstrate is either coated with a thin interlayer or patterned. Aftercool down from the growth temperature to the room temperature, it isclaimed that thermal mismatch generates defects in the substrate, not inthe GaN film, producing a thick high quality GaN material. However, theGaN material of Solomon's invention is still attached to the underlyingsubstrate, with the underlying substrate containing substantial defectsand/or cracks. Subsequently, other processing steps are required tocreate a freestanding GaN layer.

Usui et al. in U.S. Pat. No. 6,924,159 discloses a void assisted methodto manufacture GaN substrate. In this method, a first GaN thin film isdeposited on a foreign substrate, and a thin metal film such as titaniumfilm is then deposited on the first GaN thin film. The titanium metalfilm is heated in hydrogen-containing gas to form voids in the first GaNthin film. A thick GaN film is subsequently deposited on the firstvoid-containing GaN film. The voids in the first GaN film createfracture weakness, and upon cooling from the growth temperature toambient room temperature, the thick GaN layer separates itself from thesubstrate, forming a GaN wafer. However, this method requires depositionof several layers of films in different systems and cracking-separation.Thus, the process is one of low yield and high manufacturing cost.

The techniques of the prior art for manufacturing GaN wafers areattended by high manufacturing cost. There are some commercial vendorscurrently selling 2″ GaN wafers, but at very high price, reflecting thehigh manufacturing cost. Additionally, researchers have shown that thefreestanding GaN substrates formed using the laser-induced liftoffprocess can be subject to substantial bowing, which limits theirusability for device manufacturing (see, for example, “Growth of thickGaN layers with hydride vapour phase epitaxy,” B. Monemar et al., J.Crystal Growth, 281 (2005) 17-31).

In view of such prior-art approaches to forming GaN substrates, it iswell-acknowledged that there is still a need in the art for low-costmethods for producing high-quality GaN substrates.

SUMMARY

The present invention generally relates to high-quality gallium nitride(Al, Ga, In)N articles (e.g., crystals, substrates, wafers, etc.) andmethods for growing such articles.

According to one implementation, a method is provided for making a GaNarticle. An epitaxial nitride layer is deposited on a single-crystalsubstrate to form a nitride-coated substrate. A 3D nucleation GaN layeris grown on the epitaxial nitride layer by HVPE under a substantially 3Dgrowth mode. A GaN transitional layer is grown on the 3D nucleationlayer by HVPE under a condition that changes the growth mode from thesubstantially 3D growth mode to a substantially 2D growth mode. A bulkGaN layer is grown on the transitional layer by HVPE under thesubstantially 2D growth mode. A polycrystalline GaN layer is grown onthe bulk GaN layer to form a GaN/substrate bi-layer. The GaN/substratebi-layer is cooled from a growth temperature at which the bulk layer isgrown to an ambient temperature, wherein GaN material of the bi-layercracks laterally and separates from the substrate to form asubstantially crack-free free-standing GaN article.

According to another implementation, a GaN article is provided, which isproduced according to the foregoing method.

According to another implementation, the foregoing method forms aGaN/substrate bi-layer. The GaN/substrate bi-layer is cooled from agrowth temperature at which the bulk layer is grown to an ambienttemperature, wherein GaN material of the bi-layer cracks laterally andseparates from the substrate to form a substantially crack-freefree-standing GaN article.

According to another implementation, a free-standing GaN article isprovided, which is produced according to the foregoing method.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of a vertical HVPE GaN reactor.

FIG. 2 is an optical micrograph of the surface of a GaN film grown on anAlN-coated sapphire substrate. The GaN film thickness was about 1micron.

FIG. 3 is an optical micrograph of a GaN film, about 5 microns thick,grown on an AlN-coated sapphire substrate using the HVPE technique.Microcracking of the film is visible.

FIG. 4 is an optical micrograph of a pitted GaN film, about 110 micronsthick, grown on AlN-coated sapphire substrate under moderate NH₃ partialpressure (V:III ratio).

FIG. 5 illustrates plots of the pitting percentage, defined as thepercentage of area covered with pits on GaN film, versus NH₃ flow, forfurnace temperatures between 1050° C. and 1100° C. The growth rate wasabout 300 microns per hour and the film thickness was about 100 microns.

FIG. 6 illustrates plots of the calculated in-plane biaxial stresses inGaN and sapphire for a simple two-layer stress model as a function ofGaN layer thickness. Positive stress values are tensile and negativestress values are compressive. The thickness of the sapphire is 430 μmand the growth temperature is taken to be 970° C.

FIG. 7 is a schematic view of a GaN crystal structure that includes abulk GaN crystal grown on a substrate, showing different layers of thecrystal structure.

FIG. 8 is a schematic view of the GaN crystal structure illustrated inFIG. 7 after self-separation, yielding a free-standing GaN crystalseparate from the underlying substrate.

FIG. 9 is a schematic illustration of a method for producing a singlecrystal GaN wafer according to an implementation of the presentinvention.

FIG. 10 is an optical micrograph of a GaN film according to animplementation of the present invention.

FIG. 11 is an illustration of a GaN growth process, including thetemperature, NH₃ flows, and HCl flows for the nucleation, transition,bulk growth, and polycrystalline growth stages according to one exampleof the present invention.

FIG. 12 is an optical micrograph of a GaN film according to animplementation of the present invention.

FIG. 13 is an illustration of a GaN growth process, including thetemperature, NH₃ flows, and HCl flows for the nucleation, transition,bulk growth, and polycrystalline growth stages according to anotherexample of the present invention.

DETAILED DESCRIPTION

Throughout the disclosure, unless specified otherwise, certain terms areused as follows. “Single crystalline film” or “single crystal” means acrystalline structure that can be characterized with x-ray rocking curvemeasurement. The narrower the peak of the rocking curve, the better thecrystal quality. “Single crystal” does not necessarily mean that thewhole crystal is a single grain; it may contain many crystalline grainswith orientation more or less aligned. “Polycrystalline film” or“polycrystal” means that a crystal has many grains whose crystalorientations are randomly distributed. An X-ray rocking curvemeasurement of a polycrystalline film does not exhibit a peak.“Microcracks” are a cluster of localized cracks with high density ofcracks. The distance between the parallel cracks in the microcrackcluster is typically less than 100 microns. “Growth cracks” are thecracks formed during crystal growth. “Cool down cracks” or “thermalcracks” are the cracks formed after the crystal growth and during thecooling of the crystal from the growth temperature to ambient or roomtemperature. “Pits” are typically inverse pyramidal pits on the crystalsurface. “Pit-free surface” is a surface essentially having no pits onits surface. “2D growth mode” means that a growth surface remains planarand smooth during the growth. “3D growth mode” means that a growthsurface develops non-planar, three-dimensional features such as pitsduring the growth. “Pitted surface morphology” means a surface having asubstantial amount of pits on its surface. Pitted surface morphology isrelated to the 3D growth mode. “Smooth surface morphology” means that asurface is specular and has no visual defects (such as pits). Smoothsurface morphology is related to the 2D growth mode. “Nucleation layer”in some implementations may be the layer first grown on a substrate. Inother implementations, a “template layer” may be the layer first grownon a substrate. “Bulk layer” is where the majority of the crystal grown.“V:III ratio” in some implementations is the ratio of the ammonia flowto the HCl flow used during a hydride vapor phase epitaxy GaN growthprocess. “Ammonia partial pressure” is calculated according to theammonia flow, the total gas flow into a reactor, and the reactorpressure. “Growth surface” or “growing surface” or “growth front” is thesurface of the GaN crystal during the instance of the growth. “Frontside” of a GaN substrate is the growth surface side. “Back side” of aGaN substrate is the side opposite to the front side. The c-plane GaN isa polar substrate, with one surface terminated with gallium (Ga-surfaceor Ga face) and the other surface is terminated with nitrogen(nitrogen-surface or nitrogen face). In this disclosure, the“front-side” of the c-plane single crystal GaN wafer is the Ga-face.

For purposes of the present disclosure, it will be understood that whena layer (or film, region, substrate, component, device, or the like) isreferred to as being “on” or “over” another layer, that layer may bedirectly or actually on (or over) the other layer or, alternatively,intervening layers (e.g., buffer layers, transition layers, interlayers,sacrificial layers, etch-stop layers, masks, electrodes, interconnects,contacts, or the like) may also be present. A layer that is “directlyon” another layer means that no intervening layer is present, unlessotherwise indicated. It will also be understood that when a layer isreferred to as being “on” (or “over”) another layer, that layer maycover the entire surface of the other layer or only a portion of theother layer. It will be further understood that terms such as “formedon” or “disposed on” are not intended to introduce any limitationsrelating to particular methods of material transport, deposition,fabrication, surface treatment, or physical, chemical, or ionic bondingor interaction.

Unless otherwise indicated, terms such as “gallium nitride” and “GaN”are intended to describe binary, ternary, and quaternary Group IIInitride-based compounds such as, for example, gallium nitride, indiumnitride, aluminum nitride, aluminum gallium nitride, indium galliumnitride, indium aluminum nitride, and aluminum indium gallium nitride,and alloys, mixtures, or combinations of the foregoing, with or withoutadded dopants, impurities or trace components, as well as all possiblecrystalline structures and morphologies, and any derivatives or modifiedcompositions of the foregoing. Unless otherwise indicated, no limitationis placed on the stoichiometries of these compounds.

Single-crystal GaN films can be grown on sapphire substrates withvarious vapor phase growth techniques, such as molecular beam epitaxy(MBE), metal-organic vapor phase epitaxy (MOVPE), and hydride vaporphase epitaxy (HVPE). In the MBE and MOVPE growth of GaN films onsapphire, a low-temperature buffer layer is typically needed to growhigh-quality GaN film. It is not clear whether a buffer layer is neededfor HVPE GaN growth on sapphire. Lee in U.S. Pat. No. 6,528,394discloses a specific method of pre-treatment for growing GaN on sapphireusing HVPE. The pre-treatment involves etching sapphire with a gasmixture of hydrochloric acid (HCl) and ammonia (NH₃), as well asnitridation of the sapphire substrate. Molnar in U.S. Pat. No. 6,086,673discloses the use of zinc oxide (ZnO) pretreatment layer that wasfurther reacted in the gaseous environment of HCl and/or NH₃. After thistreatment of the sapphire substrate, single-crystal GaN film is thengrown by HVPE. On the other hand, Vaudo et al. in U.S. Pat. No.6,440,823 discloses the growth of a low defect density GaN layer onsapphire by an HVPE method, without using any buffer layers ornucleation layers.

Since teachings in the prior art regarding sapphire substrate treatmentor initiation prior to HVPE GaN growth are in conflict, wesystematically investigated the growth of gallium nitride film onsapphire using an HVPE process. Vertical HVPE reactors were used for theinvestigation. FIG. 1 schematically illustrates an example of a verticalHVPE reactor 100. The HVPE reactor 100 includes a quartz reactor tube104 that is heated by a multi-zone furnace 108. The reactor tube 104 isconnected to gas inlets 112, 116, and 120 for introducing reactants,carrier gases, and diluting gases. The reactor tube 104 is alsoconnected to a pump and exhaust system 124. In some implementations,inside the reactor 100, gaseous hydrochloric acid (HCl) is flowedthrough a vessel 128 containing gallium metal 132, which is at atemperature of, for example, about 850° C. The hydrochloric acid reactswith the gallium metal 132, forming gaseous GaCl, which is transportedby a carrier gas, such as nitrogen, to the deposition zone in thereactor tube 104. Ammonia (NH₃) and an inert diluent gas, such asnitrogen, are also flowed to the deposition zone where GaN crystals aredeposited. The reactor 100 is designed such that the mixing of GaCl andNH₃ does not occur near the gas outlets, ensuring no deposition of GaNon the outlets of GaCl and NH₃ and enabling long-term stability of gasflow patterns. Epi-ready c-plane sapphire substrates or other suitablesubstrates 136 may be used. The sapphire substrate 136 is placed on arotating platter 140, and heated to a temperature of, for example,900-1100° C.

A typical deposition run process is as follows: (1) a sapphire substrate136 is placed on the platter 140, (2) the reactor 100 is sealed, (3) thereactor 100 is evacuated and purged with high-purity nitrogen to removeany impurities from the system, (4) the platter 140 with the substrate136 is raised to the deposition zone, (5) the platter temperature iscontrolled at the desired deposition temperature, (6) ammonia is flowedinto the reactor 100, (7) HCl is flowed to the reactor 100 to start theGaN deposition, (8) deposition proceeds according to a predeterminedrecipe for a predetermined time, (9) the HCl and NH₃ gas flows arestopped, (10) the platter 140 is lowered and the grown crystal isgradually cooled down, and (11) the grown crystal is removed forcharacterization and further processing.

After systematically investigating the HVPE growth of GaN on sapphiresubstrates, we uncovered several issues that were not disclosed in theprior art, namely, irreproducible nucleation of single crystal GaN filmon untreated sapphire substrates, and microcracking of singlecrystalline GaN films.

GaN Nucleation on Sapphire

First, we grew various HVPE GaN films directly on sapphire substrateswithout any buffer layer or pretreatment under the conditions taught bythe prior art, i.e., a growth temperature of about 950-1050° C., a V:IIIratio (i.e., NH₃/HCl) of about 10-50, and a growth rate of about 100microns per hour. The bare sapphire substrate was heated up to thegrowth temperature, ammonia flow was turned on first to fill the reactorto a pre-determined partial pressure, and HCl flow was turned on toinitiate the growth. The GaN film grown directly on the bare sapphiresubstrate was not smooth. After analyzing the grown GaN films with x-rayrocking curve and optical microscope, we determined that the GaN filmsgrown directly on bare sapphire substrates were not single crystallinefilms. In fact, they were polycrystalline GaN. We wish not to be boundby any particular theory regarding the various results of HVPE GaNcrystal growth on sapphire, but the discrepancy in the various prior-artwork and our own work may be related to particular reactorconfigurations or surface treatments. The prior art did not teach areproducible method to grow single crystal GaN films on sapphiresubstrates by HVPE.

There is a large lattice mismatch between sapphire and gallium nitride.Furthermore, c-plane GaN is a polar crystal, i.e., one face isterminated with gallium and the opposite face of the crystal isterminated with nitrogen. On the other hand, sapphire is not a polarcrystal; the c-plane of sapphire is terminated with oxygen on bothfaces. In other GaN thin-film deposition techniques such as molecularbeam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE), a thinbuffer layer is required for high-quality single-crystalline GaN growth.The buffer layer may be an AlN layer (S. Yoshida et al., Appl. Phys.Lett., 42, 427 (1983); H. Amano et al., Appl. Phys. Lett., 48, 353(1986)) or a GaN layer grown at low temperature (S. Nakamura, Jpn. J.Appl. Phys., 30, L1705 (1991)). Lee in U.S. Pat. No. 6,528,394postulated formation of a thin AlN layer on the sapphire surface by thepre-treatment step prior to HVPE GaN growth.

U.S. Pat. No. 6,784,085, which is incorporated by reference herein inits entirety, discloses a high-temperature reactive sputtering methodfor growing high-quality AlN film on sapphire substrates. Using thismethod, we coated sapphire substrates with AlN for use as substrates forHVPE GaN growth.

High-quality GaN thin films were successfully and reproducibly grown onthe AlN-coated sapphire substrate. We first grew a thin layer of AlNfilm on a sapphire substrate by sputtering using the method disclosed inU.S. Pat. No. 6,784,085. The thickness of the AlN layer was about 0.05-2microns. X-ray rocking curve measurement indicated the AlN film wasepitaxial single-crystalline film with (0002) rocking curve full widthat half maximum (FWHM) of 50 arcsec. The AlN-coated sapphire substratewas loaded into the HVPE reactor and a GaN film was grown using theaforementioned procedure. The growth rate was about 60 microns per hour,the GaCl partial pressure was about 2.97 Torr, the NH₃ partial pressurewas about 44.6 Torr, the V:III ratio was about 15, and the growthtemperature was about 950° C. as measured with a thermocouple under theplatter. The growth time was 1 minute. The GaN film grown wastransparent with a smooth specular surface. FIG. 2 shows an opticalmicrograph of the surface of the GaN film. FIG. 2 shows a typical smoothsurface morphology for an HVPE GaN film with some hillock features.X-ray rocking curve measurements confirm the single-crystalline natureof the GaN film, with a FWHM value of 297 arcsec.

GaN Film Microcracking

After developing the nucleation of a GaN single-crystalline film on anAlN sputter-coated sapphire substrate, we investigated the growth ofthicker GaN films. We discovered a problem, namely, microcracking in theGaN films. The HVPE growth conditions were chosen to produce a smoothGaN surface. FIG. 3 shows an optical micrograph of thin GaN film, about5 microns thick on an AlN-coated sapphire substrate, grown under thesame conditions as the film shown in FIG. 2. The surface exhibits atypical smooth HVPE GaN morphology with hillock features. However,microcracks in the GaN film are apparent. The sapphire substrate remainsintact without any cracking in this case.

Because of the difference between the coefficients of thermal expansionof the sapphire substrate and the GaN film, thermal stress builds upwhen the film cools down from the typical growth temperature of about1000° C. to ambient room temperature. As discussed in open literature(for example, E. V. Etzkorn and D. R. Clarke, “Cracking of GaN Films,”J. Appl. Phys., 89 (2001) 1025), sapphire substrate shrinks faster thanGaN film during cool down, causing a compressive stress in the GaN filmdue to this thermal expansion mismatch. The compressive thermal stressin the GaN film should not cause microcracking in the GaN film duringcool down. Therefore, the microcracks must be already formed during theGaN growth and prior to cool down.

The microcracking of the GaN film during the growth suggests a tensilestress in the GaN film during the growth. We wish not to be bound by anyparticular theory regarding the origin of microcracking during GaNgrowth. However, the tensile stress may be related to the AlN layeremployed in the study, or may be related to the HVPE growth conditionused, or may be universal to the HVPE GaN growth in general. Whilecracking is noted in some instances, most prior-art teachings in HVPEGaN growth do not disclose the formation of microcracks in GaN filmduring growth. The prior art also does not teach how to eliminate themicrocracks during the HVPE GaN growth.

In order to eliminate the microcracks formed during the HVPE GaN growth,we systematically investigated GaN growth on the AlN-coated sapphireunder various growth conditions by varying growth parameters, such asGaCl flow or partial pressure (which may be determined by the flow ofhydrochloric acid (HCl)), NH₃ flow or partial pressure, growthtemperature, and associated variables such as growth rate and V:IIIratio (e.g., NH₃/HCl ratio). In this example, the V:III ratio is theratio of the NH₃/HCl flow. The growth rate is typically proportional toGaCl partial pressure, which is directly related to the HCl flow. Wefound that the surface morphology varies substantially with the growthtemperature, growth rate and ammonia partial pressure (or V:III ratio).At a constant growth temperature and GaCl partial pressure, increasingNH₃ partial pressure dramatically alters behavior of microcracking andsurface morphology. For a constant growth time (similar film thickness,about 100 microns), the HVPE GaN surface morphology gradually changesfrom a smooth, hillocked morphology with microcracks at low NH₃ partialpressure, to a surface covered with pits at moderately high NH₃ partialpressure, and eventually to polycrystalline material at high NH₃ partialpressure. When the GaN film is covered with pits, the microcracks arenot formed at all.

FIG. 4 is a micrograph of a GaN surface grown under moderately high NH₃partial pressure (moderate V:III ratio). This particular GaN film wasgrown on an AlN-coated sapphire substrate. The growth rate was about 320microns per hour, the GaCl partial pressure was around 1.8 Torr, the NH₃partial pressure was around 112.8 Torr, the V:III ratio was around 58,and the growth temperature was about 990° C. The growth time was 20minutes. Although the GaN film surface is covered with pits, the film isstill epitaxial single-crystalline film, as confirmed by x-ray rockingcurve measurement, with FWHM of 400 arcsec. The larger FWHM value of thefilm is due in part to curvature of the sample, which is known tobroaden the X-ray diffraction peak.

Similar surface morphology trends are observed with growth temperatureat otherwise constant conditions, or with growth rate at otherwiseconstant conditions. Under constant GaCl and NH₃ partial pressures(constant growth rate and V:III ratio), reducing the growth temperaturealters the growth morphology from a smooth, hillocked structure to apitted surface morphology and eventually to polycrystalline growth.Similarly, at a constant growth temperature and V:III ratio, increasingthe growth rate (by increasing both GaCl and NH₃ partial pressure)alters the surface morphology from a smooth pit-free surface to a pittedsurface and eventually to a polycrystalline morphology.

Since the pitted surface morphology eliminates the microcracks in theGaN film during the growth, we extensively studied the growth conditionsthat could yield pitted morphology. Furthermore we defined a pittingpercentage as the percentage of the surface area covered with pits.Typically for a 100-micron thick GaN film grown under constantconditions on an AlN-coated sapphire substrate, a pitting percentagegreater than 30% in the GaN film eliminates the growth microcracks. Weevaluated pitting percentage as a function of growth NH₃ partialpressure for several growth temperatures. FIG. 5 illustrates pittingpercentage for 100-micron thick GaN films grown on AlN-coated sapphiresubstrates versus the NH₃ flow for furnace temperatures ranging from1050° C. to 1100° C. The growth rate for this study was about 300microns per hour and the film thickness was about 100 microns. At lowergrowth temperature, a slight change in NH₃ partial pressure leads to alarge change in pitting percentage, whereas at higher growthtemperature, a change in NH₃ partial pressure leads to a lesser changein pitting percentage. This indicates that growth morphology is moresensitive to the NH₃ partial pressure at lower temperature than athigher growth temperature.

Thermal Stress and Cracking of GaN Grown on Sapphire

After discovering that HVPE GaN grown under conditions that yield pittedsurface morphology eliminates the microcracks in GaN during the growth,we encountered another problem: cracking of GaN and sapphire when thewafer cooled down from the growth temperature to the ambient roomtemperature due to the mismatch in the coefficients of thermalexpansion.

A sapphire substrate has a larger coefficient of thermal expansion (CTE)than a gallium nitride thin or thick film. During cooling down from agrowth temperature of, for example, about 1000° C. to room temperature,sapphire shrinks faster than the gallium nitride film. In a simpletwo-layer model, sapphire near the interface is under biaxial tensilestress and the GaN film near the interface is under biaxial compressivestress in the plane parallel to the interface. It is noted that thegallium nitride at the interface is always under biaxial compressivestress, whereas the stress on the gallium nitride surface depends on thethickness of the gallium nitride film (for a given sapphire substratethickness). The stress on the gallium nitride surface is changed fromcompressive stress to tensile stress with increased gallium nitride filmthickness. The calculated thermal stresses in GaN and sapphire for asimple two-layer model as a function of GaN layer thickness are shown inFIG. 6.

The thermal stress results in high strain energy in the GaN/sapphirebi-layer structure. The strain energy may be released by cracking.Cracking occurs, in general, if the strain is above the critical strainand energy is released by cracking. For example, when the GaN film isquite thin and sapphire is thick, the GaN/sapphire bilayer does notcrack after cooling because the GaN is under compressive stress andsapphire is under tensile stress, which is still below the criticalstress. When both the GaN layer and sapphire substrate are relativelythick, sapphire cracks under the tensile stress, which can propagateinto the GaN film, causing GaN cracking as well.

There are four main possible types of cracking when considering theGaN/sapphire bilayer structure: cracks perpendicular to the surface andcracks parallel to the surface; and cracks in the GaN and sapphire.Cracks perpendicular to the surface break the wafer. Cracks in the GaNlayer parallel to the surface (lateral cracks) can separate GaN from thesapphire substrate. Sometimes both types of crack are present, which canresult in small pieces of freestanding GaN.

Methods for Producing GaN Wafer by Self-Separation

U.S. Provisional Patent Application Ser. No. 60/750,982, filed Dec. 16,2005, titled “METHOD OF PRODUCING FREESTANDING GALLIUM NITRIDE BYSELF-SEPARATION”, which is incorporated by reference herein in itsentirety, discloses a method for making large-area freestanding GaNsubstrates. According to this method, a thick GaN/substrate bi-layer isproduced in a growth sequence comprising several growth conditions. Thegrowth steps include depositing an epitaxial nitride layer on thesubstrate, growing a thin GaN layer on the nitride-coated substrateunder a 3D growth mode that results in a surface covered with pits,growing a transitional GaN layer on the 3D growth layer to recover froma heavily pitted surface morphology to a less pitted surface morphology,and growing a bulk growth layer on the recovery layer. The resultingGaN/substrate bi-layer is then cooled from the growth temperature toroom temperature whereby the GaN layer cracks laterally and separatesfrom the underlying substrate by itself during cooling from the growthtemperature down to room temperature.

The GaN growth method according to this example will now be describedwith reference to FIG. 7, which schematically illustrates a GaN crystalstructure 700, and FIG. 8, which schematically illustrates a GaNcrystal/substrate bi-layer 800 after self-separation.

Referring to FIG. 7, a suitable substrate 704 is provided. In someimplementations, the substrate 704 may have a characteristic dimension(e.g., diameter) of about 2 inches or greater. As further examples, thediameter of the substrate 704 may be about 3″ or greater, about 4″ orgreater, or any other suitable size such as about 12″ or greater. Thesubstrate 704 may be sapphire (Al₂O₃), although other suitablesingle-crystal substrates 704 may be utilized. Non-limiting examples ofsuitable substrates 704 include sapphire, silicon carbide, galliumarsenide, zinc oxide, silicon, spinel, lithium gallate, lithiumaluminate, etc.

The first step of the growth process is to deposit a thin epitaxialnitride layer 708 on the substrate 704. The purpose of this epitaxialnitride layer 708 is to provide a template for epitaxial growth of GaN.The epitaxial nitride layer 708 in one embodiment is prepared byhigh-temperature reactive sputtering in a sputtering chamber. Theepitaxial nitride layer 708 may also be formed by molecular beam epitaxy(MBE), metal-organic vapor phase epitaxy (MOVPE or MOCVD), hydride vaporphase epitaxy, or high-temperature annealing of the substrate 704 inammonia. In one example, the thickness of the epitaxial nitride layer708 is in the range (ranges) from about 0.05 to about 10 microns. Inanother example, the thickness of the epitaxial nitride layer 708 rangesfrom about 0.2 to about 2 microns. Other types of template layers may beused, for example, a GaN or AlGaN layer, grown by MOVPE, MBE or HVPE.

The second step of the growth process is to grow a nucleation GaN layer712 by hydride vapor phase epitaxy in a 3D growth mode with a growthcondition that yields a pitted surface morphology. The growth conditionfor this layer 712 is typically higher growth rate, and/or higherammonia flow (or V:III ratio), and/or lower growth temperature than the“optimal” thin-film growth condition. The “optimal” thin-film growthcondition is one that would produce smooth, substantially pit-free,crack-free thin films (e.g., with a thickness less than 3 microns), butwould produce microcracked thick films (e.g., with a thickness equal toor greater than 20 microns). As one specific example of an optimizedgrowth condition, a 1-micron thick GaN film that is transparent and hasa smooth specular surface has been grown on an AlN-coated sapphiresubstrate by the inventors. The growth rate was about 60 microns perhour, the GaCl partial pressure was about 3 Torr, the NH₃ partialpressure was about 45 Torr, the V:III ratio was about 15, the growthtemperature was about 950° C., and the growth time was one minute. Whengrowing a thin film (≦3 μm), this “optimal” thin-film growth conditiontypically produces a crack-free film, whereas when growing a thick film(≧20 μm), the “optimal” growth condition typically produces amicrocracked film.

There are two purposes for this pitted growth layer 712: first is toprevent microcracking of GaN during the growth, and second is to createin the GaN film a certain stress condition that will facilitate thelateral cracks during cool down. In one example, the thickness of the 3Dgrowth layer 712 may range from about 5 to about 500 microns. In anotherexample, the thickness of the 3D growth layer 712 may range from about 5to about 200 microns. In another example, the thickness of the 3D growthlayer 712 may range from about 5 to about 100 microns. In anotherexample, the thickness of the growth layer 712 ranges from about 10 toabout 50 microns. In yet another example, the thickness of the 3D growthlayer 712 ranges from about 20 microns to about 30 microns. In anotherexample, the thickness of the 3D growth layer 712 is about 20 microns.

The third step of the growth process is to change the growth conditionsto recover the surface morphology from a highly pitted surfacemorphology to a less pitted surface morphology, which prepares thesurface of the growing crystal for the subsequent bulk growth stagewhere most of the GaN crystal is grown. In this third step, the growthmode is gradually changed from a 3D growth mode to a 2D growth mode.This transition is accomplished by growing a recovery layer 716 on the3D growth layer 712 under conditions such as lower growth rate, and/orlower ammonia partial pressure, and/or higher growth temperature thanthe growth condition of the 3D nucleation layer 712. In one example, thethickness of the morphology recovery layer 716 may range from about 5 toabout 100 microns. In another example, the thickness of the recoverylayer 716 ranges from about 5 to about 50 microns. In another example,the thickness of the recovery layer 716 is about 8 microns. The purposesof the recovery layer 716 are to prevent the GaN film from turning intopolycrystalline, and to obtain a film stress state that facilitateslateral cracks during cool down.

The fourth growth step is the bulk growth step where the bulk of the GaNfilm is grown. As illustrated in FIG. 7, a bulk layer or crystal 720 isgrown on the recovery layer 716. The growth condition is chosen so thatthe morphology of the GaN film is slightly pitted or pit-free. The GaNgrowth mode in this step is substantially a 2D growth mode. The growthconditions may be held constant during this step. Alternatively, thegrowth condition may be slightly ramped, for example, slightly rampingdown ammonia flow, slightly ramping down the growth rate or slightlyramping up the temperature. The purpose of the ramping in the bulkgrowth step is to further reduce the density of the pits on the growingGaN surface. During the bulk growth step, the density of the pits on thegrowing GaN surface is gradually reduced. At the end of the bulk growth,the GaN surface is slightly pitted or pit-free. In one example, thethickness of the GaN bulk layer 720 grown in the bulk growth step rangesfrom about 500 to about 2000 microns (0.5 to 2 mm). In another example,the thickness of the GaN bulk layer 720 ranges from about 1000 to about1500 microns (1 to 1.5 mm). In some implementations, the crystal growthprocess is performed to yield a single wafer. In other implementations,the process may be performed to grow a GaN boule that can be sliced intomultiple wafers, in which case a thicker bulk layer 720 may be grown,for example, about 2 mm or greater, from about 2 mm to about 10 mm, orabout 10 mm or greater.

After completing the growth, the resulting thick GaN-on-substratebi-layer is gradually cooled down. In one example, the cooling rate isless than about 20° C. per minute, whereas in another example it is lessthan about 10° C. per minute. In another example, the rate of cooling isabout 6° C. per minute. During this cool down time, lateral crackingoccurs in the GaN film with the crack plane substantially or essentiallyparallel to the GaN/substrate interface, leading to the separation ofGaN from the underlying substrate.

FIG. 8 illustrates the resulting separated GaN/substrate bi-layerstructure 800. A thick GaN wafer 822 having a characteristic dimension(e.g., diameter) as large as the initial substrate 704 (FIG. 7) may beobtained, along with the substrate 806 covered with a thin layer of GaN.As examples, when a 2″ substrate 704 is utilized, a 2″ GaN wafer 822 maybe obtained. When a 3″ substrate 704 is utilized, a 3″ GaN wafer 822 maybe obtained. The substrate 806 may remain intact, or remain partiallyintact with edge fracture, or fracture into several pieces. Theremaining GaN on the substrate 806 is typically less than 500 micronsthick. The thickness of the freestanding GaN wafer 822 typically rangesfrom about 0.5 mm to about 10 mm.

After self-separation, the GaN wafers may be mechanically polished to aspecified wafer thickness. To remove the subsurface damage, the wafermay be chemically mechanically polished as the last step. Reactive ionetching or inductively coupled plasma etching may also be used to removethe damaged surface layer. Other suitable surface finishing techniquesmay alternately or additionally be employed.

When implementing this method, the lateral cracks occur in theGaN/substrate bi-layer structure 800 because it is the most effectiveway to relieve the thermal stress. We wish not to be bound by anyparticular theory regarding how the lateral cracks occur, but here wepresent a possible mechanism by which lateral cracking in the bi-layerstructure 800 may occur. Since a substrate material such as sapphireshrinks more than GaN during cool down, the thermal stress condition ofthe GaN/substrate bi-layer structure 800 results in the substrate beingunder tensile stress whereas the GaN near the interface is under thecompressive stress. Film fracture behavior under compressive stress hasbeen reported in the open literature (see, for example, “Fracture inThin Films,” S. Zuo, Encyclopedia of Materials: Science and Technology,Elsevier Science, 2001) and may be used to describe the GaN/substratesystem. In accordance with the present method, when a film is undercompression, it can self-separate, or debond, from the underlyingsubstrate.

In general, fracture is driven by the relaxation of residual stresses,in this case, thermal stress due to expansion mismatch. Fracture willoccur if the driving force exceeds the fracture resistance for theparticular fracture mechanism. Cracks can form at pre-existing flaws inthe film, in the substrate, or at areas where the resolved stress isconcentrated and exceeds the critical fracture value. The mechanics andprocess of the fracture processes are not fully understood, but underbiaxial compressive stress, the in-plane lattice constants a and b ofthe GaN film are shortened whereas the lattice constant c is elongated.The increased lattice constant c under the biaxial stress weakens thebond strength causing the separation in the c direction, i.e., lateralcracking. This stress may be increased locally by geometric factors.

In the present case, the structure of the as-grown GaN layer(s) leads toconditions conducive to compressive debonding at or near the interfacebetween the GaN and substrate. The epitaxial nitride (e.g., AlN) layer,the first GaN nucleation step, the second GaN nucleation step, or somecombination of the three, results in the initiation of the debondingbehavior during cool down. Experimentally, the initial debonding hasbeen observed to occur near the center of the substrate bi-layer. Afterdebonding, buckling occurs as the self-separated area increases. As theelastic constants of the GaN and the substrate (e.g., sapphire) areclose in magnitude and plastic deformation is extremely small, thedegree of buckling is limited. After buckling, both normal and shearstresses develop that will continue to grow the debonded area like acrack. Under preferred conditions, the debonding continues to the edgeof the GaN/substrate bi-layer and separates the GaN layer from thesubstrate.

There are three major competing processes that have also been observedfor the relief of the thermal stress in the GaN/substrate bi-layer: (1)lateral cracking in the GaN layer due to compressive stress in the GaNlayer near the GaN/substrate interface, to form a whole piece of thickstress-free GaN wafer and a thinner-GaN/substrate bi-layer, (2) verticalcracks in sapphire due to tensile stress developed during cool down (thevertical cracks may also propagate to GaN), and (3) the presence of bothlateral cracking in GaN and vertical cracking in substrate and GaN. Acrack will occur in the direction where the stress exceeds the criticalstress.

In addition to the thermal stress during cool down, the GaN/substratebi-layer also experiences a growth stress built up during the crystalgrowth. The growth stress can lead to GaN microcracks under certaingrowth conditions as discussed in the previous paragraphs. The growthstress can also lead to the breaking of the substrate and the GaN layerduring the growth, which is observed under non-optimal growthconditions.

The growth sequence employed prepares in the GaN layer a certain stressstate during the growth, such as increased compressive stress in the GaNlayer near the interface or reduced tensile stress on the substrate,enabling the compressive stress debonding during cool down whilepreventing growth stress leading to microcracking or vertical crackingin the substrate or GaN layer, yielding a freestanding GaN substrate.The formation of the 3D nucleation layer is one key aspect for theseparation of the bulk GaN layer during cool down. If the firstnucleation layer is grown under conditions of 2D growth, the most likelymechanism for thermal stress relief for the thick GaN/substrate bi-layeris vertical cracking in the substrate that propagates to the GaN layer.

Methods for Producing GaN Articles with the Use of a PolycrystallineLayer

In some cases, the above-described method for making a freestandingsubstrate may suffer a drawback; namely, GaN may also break during theseparation, reducing the yield of the process.

According to the present disclosure, the method of GaN substrateproduction disclosed in above-described U.S. Prov. Pat. App. Ser. No.60/750,982 may be further improved by introducing a mechanicallystronger polycrystalline layer to cap the single-crystalline GaN layer,thus improving the yield of the process.

FIG. 9 is a schematic, sequential illustration of an example of aprocess 900 of the present invention. First, a sapphire substrate 904 isprovided. An epitaxial nitride (e.g., AlN) layer 908 is then depositedon a surface 906 of the sapphire substrate 904. The deposition ofepitaxial nitride layer 908 may be done in the same reactor as for thesubsequent GaN growth, or in a different deposition chamber. GaNmaterial is subsequently deposited on the nitride-coated substrate904/908 by hydride vapor phase epitaxy in multiple steps with differentgrowth conditions for each step. A first GaN layer 912 is grown under acondition that results in a pitted surface morphology, and suchconditions are characterized by relatively higher growth rate, and/orhigh ammonia flow, and/or lower growth temperature than the optimalthin-film growth conditions that would yield a smooth surfacemorphology. The first GaN layer 912 thus has a pitted surface 914. Asecond GaN layer 916 is grown from the first GaN layer 912. The secondGaN layer 916 functions as a transitional layer that is grown under acondition that gradually fills the pits and yields a much less pittedGaN surface 918, and such growth conditions are characterized byrelatively lower growth rate, and/or lower ammonia flow, and/or highergrowth temperature than the first pitted growth step. A third GaN layer920 is then grown on the second GaN layer 916. The third GaN layer 920is the bulk layer where the majority of single-crystal GaN is grown. Afourth GaN layer 924 is then grown on the third GaN layer 920. Thefourth GaN layer 924 is a polycrystalline GaN layer that is provided toincrease the overall mechanical strength of the entire GaN layers.

During cooling down from the growth temperature to ambient roomtemperature, the grown GaN film separates from the sapphire substrate904 via lateral cracking, producing a free-standing crack-free GaNarticle 932 that includes a single-crystal layer 936 and thepolycrystalline layer 924. The polycrystalline GaN material 924 ismechanically stronger than the single-crystal layer 936 and reduces thebreakage of the GaN article 932. The freestanding crack-free GaN article932 is processed by removing the polycrystalline GaN layer 924 to yielda single crystal GaN wafer 940.

Continuing with the example illustrated in FIG. 9, in someimplementations, the substrate 904 may have a characteristic dimension(e.g., diameter) of about 2 inches or greater. As further examples, thediameter of the substrate 904 may be about 3″ or greater, about 4″ orgreater, or any other suitable size such as about 12″ or greater. Thesubstrate 904 may be sapphire (Al₂O₃), although other suitablesingle-crystal substrates 904 may be utilized.

The surface 906 of the substrate 904 may be exactly c-plane or vicinalsurfaces of the c-plane. Vicinal surfaces may promote step-flow duringthe HVPE GaN growth and may yield smoother surface morphology. Theoffcut angle of the vicinal surface with respect to the c-plane is inone example between about 0.1° and about 10°, and in another examplebetween about 0.5° and about 5°. The direction of offcut may be alongthe <1-100> direction or along the <11-20> direction, or along adirection between <1-100> and <11-20>.

In one implementation, the epitaxial nitride layer 908 is deposited byreactive sputtering on a heated substrate in a sputter depositionchamber. The nitride-coated substrate 904/908 is subsequently removedfrom the sputter chamber and loaded into the HVPE reactor for GaNgrowth. Other nitride nucleation layers such as, for example, MN grownby MOCVD or MBE, GaN grown by MOCVD or MBE, AlGaN grown by MOCVD or MBE,or the like may also be used. In some implementations, a reactivesputtering-deposited AlN layer has an advantage of lower cost thanMOCVD- or MBE-deposited nitride layers. AlN layers may also be grown inthe HVPE reactor by incorporating an Al source so that hydrochloric acidreacts with Al to form aluminum chloride, which reacts with ammonia inthe deposition zone to form AlN on the substrate surface. In oneexample, the thickness of the epitaxial nitride layer 908 is in therange (ranges) from about 0.05 to about 10 microns. In another example,the thickness of the epitaxial nitride layer 908 ranges from about 0.5to about 2 microns. In another example, the thickness of the epitaxialnitride layer 908 ranges from about 0.5 to about 2 microns.

In some implementations, depending on such factors as process conditionsand the specific reactor being employed, the deposition of the epitaxialnitride layer 908 may be desirable for growing single-crystal GaN filmson sapphire substrates using the HVPE process. In other HVPE reactorsystems, however, the single-crystalline GaN layer 912 may be growndirectly on the substrate 904 by HVPE without using a template layersuch as the epitaxial nitride layer 908 prior to HVPE growth. In suchcases, the growth sequence according to the teaching of the presentinvention can skip the nitride-coating step and proceed directly to thenext growth step.

The second step of the growth process is to grow an epitaxial GaN layer912 by hydride vapor phase epitaxy in a 3D growth mode. The uncoatedsubstrate 904 or nitride-coated substrate 904/908 is loaded into an HVPEreactor, and the reactor may be purged with high-purity nitrogen toremove impurities. An epitaxial layer 912 of gallium nitride is thengrown. This GaN layer 912 is grown in a three-dimensional (3D) growthmode, where the surface 914 of the film is very rough and covered withpits. If the growth is stopped at the end of the first step and wafertaken out of the reactor, the GaN surface 914 is not specular, as shownin a microphotograph in FIG. 10. The GaN film at this stage is stillsingle-crystalline film as demonstrated by x-ray rocking curvemeasurements. The pit coverage, defined as the percentage of the surfacecovered with the pits on the surface, is in one example greater thanabout 50%, and in another example greater than about 75% at the end ofthis step. In other examples, the pitting percentage is greater thanabout 90%. The growth condition for this layer 912 is typically highergrowth rate, and/or higher ammonia flow (V:III ratio), and/or lowergrowth temperature than the “optimal” thin-film growth condition. Anexample of an “optimal growth condition” is an HVPE GaN growth conditionthat would yield a pit-free GaN thin film (e.g., ≦3 μm) on an AlN-coatedsapphire substrate.

There are two purposes for this 3D growth layer 912: first is to preventmicrocracking of GaN during the growth, and second is to present the GaNfilm with a certain stress condition that will facilitate the lateralcracks during cool down. In one example, the thickness of the 3D growthlayer 912 may range from about 5 to about 100 microns. In anotherexample, the thickness of the growth layer 912 ranges from about 10 toabout 50 microns. In yet another example, the thickness of the 3D growthlayer 912 ranges from about 20 microns to about 30 microns. In anotherexample, the thickness of the 3D growth layer 912 is about 20 microns.

In one implementation, the growth temperature in the 3D growth moderanges from about 900° C. to about 1000° C., the V:III ratio in the 3Dgrowth mode ranges from about 10 to about 100, and the growth rate inthe 3D growth mode ranges from about 50 μm/hr to about 500 μm/hr.

If the GaN film is grown under the 3D growth mode with higher thickness,the GaN film quality is gradually changed from an epitaxialsingle-crystalline film to a polycrystalline film.

The third step of the growth process is to change the HVPE growthconditions to transition the surface morphology from a heavily pittedsurface morphology to a gradually less pitted surface morphology. Thetransition layer 916 is grown under conditions such as lower growthrate, and/or lower NH₃ flow, and/or higher growth temperature than thegrowth condition of the nucleation layer 912. In one example, thethickness of this morphology transition layer 916 ranges from about 5 toabout 500 microns. In another example, the thickness of the transitionlayer 916 ranges from about 5 to about 200 microns. In another example,the thickness of the transition layer 916 ranges from about 5 to about100 microns. In another example, the thickness of the transition layer916 ranges from about 5 to about 50 microns. In another example, thethickness of the transition layer 916 is about 8 microns. The purposesof the transition layer 916 are to prevent the GaN film from turninginto polycrystalline, and to prepare in the film a stress state thatfacilitates lateral cracks during cool down. At the end of growth of thetransitional layer 916, the growth surface 918 is substantiallypit-free. The surface coverage of pits at the growth surface 918 aftergrowing the transitional layer 916 is in one example less than about10%, in another example less than about 5%, and in another example lessthan about 1%.

In one implementation, the growth temperature in the recovery layergrowth mode ranges from about 920° C. to about 1100° C., the V:III ratioin the recovery layer growth mode ranges from about 8 to about 80, andthe growth rate in the recovery layer growth mode ranges from about 50μm/hr to about 500 μm/hr.

The fourth growth step is the bulk growth step where the bulk of thesingle-crystalline GaN film is grown. The growth condition is chosen sothat the morphology of the GaN film remains slightly pitted or pit-free.The GaN growth mode in this step is substantially a 2D growth mode. Thegrowth conditions may be held constant during this step. Alternatively,one or more growth conditions may be slightly ramped, for example,slightly ramping down ammonia flow or slightly ramping down the growthrate or slightly ramping up the temperature. The purpose of the rampingin the bulk growth step is to further reduce the density of the pits onthe growing GaN surface. During the bulk growth step, the density of thepits on the growing GaN surface is gradually reducing. At the end of thebulk growth, the GaN surface 922 is slightly pitted or pit-free. In oneexample, the thickness of the GaN bulk layer 920 grown in the bulkgrowth step ranges from about 500 to about 2000 microns (about 0.5 toabout 2 mm). In another example, the thickness of the GaN bulk layer 920ranges from about 1000 to about 1500 microns (about 1 to about 1.5 mm).

In one implementation, the growth temperature in the bulk growth stageranges from about 950° C. to about 1100° C., the V:III ratio in the bulkgrowth stage ranges from about 5 to about 50, and the growth rate in thebulk growth stage ranges from about 50 μm/hr to about 500 μm/hr.

The fifth step is to grow a polycrystalline GaN film 924 on top of thesingle-crystalline GaN film 920 grown in the fourth step by changing thegrowth condition. The polycrystalline GaN film 924 may be grown byreducing the growth temperature, and/or increasing the NH₃ flow, and/orincreasing the growth rate, as compared with the growth conditionsemployed in the preceding bulk growth step. In one example, thethickness of the polycrystalline GaN film 924 ranges from about 500 toabout 2000 microns (0.5 to 2 mm).

In one implementation, the growth temperature in the polycrystallinegrowth stage ranges from about 850° C. to about 1000° C., the V:IIIratio in the bulk growth stage ranges from about 20 to about 200, andthe growth rate in the bulk growth stage ranges from about 100 μm/hr toabout 500 μm/hr.

The polycrystalline layer 924 is postulated herein to improve theintegrity of the GaN article 932 in the following manners. In a singlecrystal, the cleavage planes of the crystal are substantially alignedand when a crack forms, there is little to blunt the crack fromprogressing through the thickness or length of the crystal. When apolycrystalline layer 924 is deposited on the single crystal, thecrystal cleavage planes in the polycrystal are not aligned with theplanes in the single crystal. If a crack were to propagate through thesingle crystal, the energy required to propagate the crack through thepolycrystal increases, thus making the article stronger. Additionally,when a polycrystalline layer 924 is deposited on top of the singlecrystal, the thickness of the crystal changes and the stress in theentire GaN layer due to thermal expansion mismatch is reduced. Thisreduces the likelihood of crack formation in the article.

After completing the growth, the thick GaN-on-substrate bi-layer isgradually cooled down. The cooling rate is in one example less thanabout 20° C. per minute, and in another example less than about 10° C.per minute. In another example, the rate of cooling is about 6° C. perminute. During this cool down time, lateral cracking occurs in the GaNfilm with the crack plane essentially parallel to the GaN/substrateinterface, leading to the separation of GaN from the underlyingsubstrate. A thick GaN article 932 having a characteristic dimension(e.g., diameter) as large as the initial substrate 904 may be obtained,along with the substrate covered with a thin layer of GaN. As examples,when a 2″ substrate 904 is utilized, a 2″ GaN article 932 may beobtained. When a 3″ substrate 904 is utilized, a 3″ GaN article 932 maybe obtained. When a 4″ substrate 904 is utilized, a 4″ GaN article 932may be obtained. The substrate 904 may remain intact, or remainpartially intact with edge fracture, or fracture into several pieces.The remaining GaN on the substrate 904 is typically less than 500microns thick. The resulting freestanding GaN article 932 is typically1-4 mm thick.

The freestanding GaN article 932 may be processed into asingle-crystalline GaN wafer or substrate 940 by mechanical means suchas grinding or lapping and polishing. In one example, the freestandingGaN article 932 is first sized into a desired wafer shape (hereindefined as a wafer blank), optionally with major and minor flats toindicate the crystal orientation of the substrate 940. In one example,the sized GaN wafer blank is about 10 mm×10 mm square or greater—i.e.,the sized GaN wafer blank includes a side having a length of about 10 mmor greater. In another example, the sized GaN wafer blank is about 18mm×18 mm square. In another example, the sized GaN wafer blank is about1 inch round or greater—i.e., the sized GaN wafer blank is circular andhas a diameter of about 1 inch or greater. In another example, the sizedGaN wafer blank is about 2 inches round or greater. The front of thewafer blank is the polycrystalline GaN and the back of the wafer blankis the nitrogen-face of the single-crystalline GaN. The polycrystallinepart of the wafer blank may be removed by mechanical means such asgrinding and/or lapping. The thickness removed from the front side ofthe wafer blank may be at least as much as or greater than the thicknessof the polycrystalline material to expose the Ga-face 944 of the singlecrystalline GaN substrate 940. After completely removing thepolycrystalline material from the front side of the wafer blank, thefront side (Ga face) may be further polished with diamond slurry. TheGa-surface 944 may be finished with a chemical-mechanical polish stepthat removes the surface and subsurface damage and produces an epi-readysurface. The back side of the wafer blank may be processed by mechanicalmeans such as grinding or lapping to planarize (mechanically flatten)the surface and to achieve the desired wafer thickness. Since thecrystal defect density is reduced during the growth ofsingle-crystalline GaN, it may be preferable to take away material fromthe back side to achieve the desired wafer thickness. Optionally, theback side may be polished to produce an optical finish.

In some implementations, during the bulk growth, no impurity isintroduced and all the gas sources are purified, in order to achievehigh-purity GaN crystals. In some examples, the impurity concentrationin the high-purity GaN layer is less than about 10¹⁷ cm⁻³. In otherexamples, the impurity concentration is less than about 10¹⁶ cm⁻³. Inother examples, the impurity concentration is less than about 10¹⁵ cm⁻³.

Alternatively, in other implementations, during the bulk growth, n-typeimpurities, such as silicon (introduced, for example, as diluted silane)or oxygen, are introduced to produce n-type GaN crystals. Theintroduction of impurities may occur at any stage of the GaN growth. Insome examples, the electron concentration in the n-type bulk GaN layeris greater than about 10¹⁷ cm⁻³. In other examples, the electronconcentration is greater than about 10¹⁸ cm⁻³. In other examples, theelectron concentration is greater than about 10¹⁹ cm⁻³. In someexamples, the resistivity of the n-type GaN layer is less than about 0.1ohm-cm. In other examples, the resistivity is less than about 0.01ohm-cm. In other examples, the resistivity is less than about 0.001ohm-cm. Conductive GaN wafers may be used as substrates for thefabrication of optoelectronic devices such as light emitting diodes,laser diodes, and photodetectors. Conductive GaN wafers may also be usedas substrates for electronic devices such as Schottky diodes andheterojunction bipolar transistors.

In other implementations, during the bulk growth, p-type impurities suchas magnesium (Mg) are introduced to produce p-type GaN crystals. Mg maybe introduced as metal-organic compound or as metal vapor. Theintroduction of impurities may occur at any stage of the growth. In someexamples, the hole concentration in the p-type bulk GaN layer is greaterthan about 10¹⁷ cm⁻³. In other examples, the hole concentration isgreater than about 10¹⁸ cm⁻³. In other examples, the hole concentrationis greater than about 10¹⁹ cm⁻³. In some examples, the resistivity ofthe p-type GaN layer is less than about 0.1 ohm-cm. In other examples,the resistivity is less than about 0.01 ohm-cm. In other examples, theresistivity is less than about 0.001 ohm-cm.

In other implementations, the electrical properties of the bulk GaNcrystal can also be made semi-insulating by introducing a deep-levelacceptor. Transition metals, such as iron, are deep level acceptors.Iron can be introduced to the deposition zone either via a metal-organiccompound such as ferrocene or via iron chloride formed by reacting ironwith hydrochloric acid. The concentration of the transition metal in thebulk GaN layer may range from about 10¹⁶ to about 10²⁰ cm⁻³. In otherexamples, the concentration of the transition metal ranges from about10¹⁷ to about 10¹⁹. In other examples, the concentration of thetransition metal is around 10¹⁸ cm⁻³. The resistivity of the SI bulk GaNlayer at room temperature may be greater than about 10⁶ ohm-cm. In otherexamples, the resistivity is greater than about 10⁷ ohm-cm. In otherexamples, the resistivity is greater than about 10⁸ ohm-cm.Semi-insulating GaN wafers can be used as substrates for electronicdevices such as high electron mobility transistors.

Certain implementations of the present invention may be furtherunderstood by the following illustrative, non-limiting examples.

Example 1 Si-Doped GaN Growth

In this example, we illustrate production of silicon-doped 2″ singlecrystalline GaN wafer. A 2″ c-plane sapphire was used as the startingsubstrate. An epitaxial AlN layer was deposited using high temperaturereactive sputtering method, as disclosed in U.S. Pat. No. 6,704,085. Thethickness of the AlN layer was approximately 0.25 μm. X-ray rockingcurve measurement showed a full width at half maximum of about 55 arcsecfor the AlN(0002) reflection, indicating high crystal quality of the AlNfilm. The AlN-coated sapphire substrate was loaded into the previouslydescribed vertical HVPE system and the GaN growth was commenced.

FIG. 11 is an illustration of the HVPE GaN growth process 1100 for thisexample, including the temperature 1104, NH₃ flows 1108, and HCl flows1112 for the nucleation stage 1122, transition stage 1126, bulk growthstage 1130, and polycrystalline growth stage 1134. The growth processincluded a nucleation step 1122 on the AlN-coated sapphire substratewith a rough surface morphology (3D growth mode), transitioning 1126 thegrowth condition from the nucleation stage (3D growth mode) 1122 to thesingle-crystalline bulk growth stage (2D growth mode) 1130,single-crystalline bulk growth 1130, and polycrystalline cap growth1134.

The nucleation layer was grown at a growth temperature of about 995° C.,HCl flow of about 120 sccm, and NH₃ flow of about 4000 sccm,corresponding to a growth rate of about 330 μm/hr. The growth time wasabout 5 minutes, corresponding to a thickness of about 28 μm. In someexamples, the growth was stopped at this stage and the wafer was takenout for examination. The GaN surface was very rough and not specular.Under optical microscope examination, the surface contains high densityof pits, similar to the image shown in FIG. 10.

After the growth of the 3D nucleation layer, the growth condition waschanged to influence the evolution of the surface morphology from aheavily pitted surface to a much less pitted surface. The growthtemperature was raised by about 15° C. to about. 1010° C., NH₃ flow wasreduced from about 4000 sccm to about 2000 sccm. The growth rate wasremained at 330 μm/hr and the growth time was 30 minutes, correspondingto a nominal thickness of about 165 μm for the transitional layer.Silane was introduced into the reactor during the growth. In someexamples, the growth was stopped at this stage, and the wafer was takenout of the reactor for examination. The GaN surface was visuallyspecular. When examined under optical microscope, the GaN surfaceexhibited typical hillock morphology with some pits. FIG. 12 shows anoptical micrograph of the GaN film grown at this stage. The surface areaoccupied by the pits was quite low, typically less than 5%.

Following the transition layer, the single crystalline bulk GaN layerwas grown. The growth temperature and the growth rate remained the sameas those for the transitional layer, 1010° C. and 330 μm/hr. The NH₃flow was further reduced from 2000 sccm to 1600 sccm for this stage ofthe growth. The silane flow was not changed. The growth time for thisstep was 3.5 hours, corresponding to a nominal thickness of about 1.1 mmfor the material grown in this step. The total epitaxialsingle-crystalline GaN film grown was about 1.35 mm thick. In someexamples, the growth was stopped at this stage, and the wafer was takenout of the reactor for examination. The surface morphology was similarto that shown in FIG. 12, except that the hillock feature is larger andsurface is almost pit-free. Under these growth conditions, the GaNcracked laterally during cool down and separated the GaN from thesapphire (the sapphire still has a thin GaN layer). Because the GaN alsohas a few cracks from the fracture during cool-down, frequently onlylarge pieces of freestanding GaN and not the whole 2″ substrate wereobtained.

After the growth of the bulk single crystalline GaN layer, the growthcondition was changed to grow a mechanically stronger polycrystallineGaN cap. The growth temperature was reduced by 20° C. to 990° C. and theNH₃ flow was increased from 1600 sccm to 6000 sccm. The HCl flow waskept at 120 sccm for a nominal growth rate of 330 μm/hr. At this growthcondition, the GaN film gradually changed into a polycrystalline filmduring the growth. In this example, the growth time for thepolycrystalline cap was about 4 hours and the nominal thickness of thepolycrystalline GaN material was about 1.3 mm. The total GaN thicknesswas about 2.6 mm.

After completion of the growth sequence, the GaN/sapphire bi-layer wascooled to room temperature at a cooling rate of approximately 6° C. perminute. During the cooling process, the GaN cracked laterally, forming a2″ freestanding GaN article and the sapphire substrate with a thin GaNlayer still attached. Because of the presence of the mechanicallystronger polycrystalline GaN layer, the GaN article did not crackvertically during the laterally cracking separation. The freestanding 2″GaN article was crack-free. The thickness of the freestanding GaNarticle was about 2.1 mm.

The top surface of the freestanding GaN was polycrystalline material andthe bottom surface was the nitrogen face (N-face) of the singlecrystalline GaN. The bottom surface was not flat because it wasgenerated from lateral cracking.

The 2″ freestanding GaN article was processed into 2″ GaN wafer. First,edge grinding was performed to make into within 0.005″ tolerance of the2″ wafer diameter. Because the initial sapphire had a major flat, thegrown GaN article also had a flat. The ground 2″ GaN article is called awafer blank herein.

The 2″ GaN wafer blank was mounted on a fixture using wax with the frontsurface facing the fixture. The back surface (N-face) was lapped on ametal plate with diamond slurry until a uniform surface finish wasachieved. At this stage, the thickness of the blank was about 1.8 mm.The 2″ GaN wafer blank was removed from the fixture and mounted againusing wax with the back side facing the fixture. The front side waslapped on a metal plate with diamond slurry. About 1.3 mm of the frontmaterial was removed. At this stage, the polycrystalline material wascompletely removed, resulting in a 2″ single crystalline GaN wafer about500 micron thick. Because silicon doping was used during the growth, theGaN wafer was conductive with a resistivity less than 0.05 ohm-cm.

Example 2 Semi-Insulating GaN Growth

In this example, we illustrate production of semi-insulating 2″ GaNsubstrate. Sapphire(0001) was used as the starting substrate. An AlNlayer approximately 0.25 μm thick was grown on the sapphire substrateusing reactive sputtering on the heated substrate. X-ray diffraction wasused to verify the AlN film was single crystal with a full width at halfmaximum of 55 arcsec. The AlN/sapphire structure was loaded into thepreviously described vertical HVPE system and the GaN growth wascommenced.

The HVPE GaN film was grown by a multiple-step method. FIG. 13 is anillustration of the HVPE GaN growth process 1300 for this example,including the temperature 1304, NH₃ flows 1308, and HCl flows 1312 forthe nucleation stage 1322, transition stage 1326, bulk growth stage1330, and polycrystalline growth stage 1334. The growth process 1300included a nucleation step 1322 on the AlN-coated sapphire substratewith a rough surface morphology (3D growth mode), transitioning 1326 thegrowth condition from the nucleation stage (3D growth mode 1322) to thesingle-crystalline bulk growth stage (2D growth mode) 1330,single-crystalline bulk growth 1330, and polycrystalline cap growth1334.

The GaN film was first grown under conditions of growth rate ofapproximately 260 microns per hour, growth temperature of 968° C., HClflow rate of 92 sccm, and NH₃ flow rate of 2000 sccm. This was the GaNnucleation layer on the AlN-coated surface. The growth time for thenucleation layer was approximately 4 minutes and the thickness of thislayer was approximately 18 microns. In some runs, the growth was stoppedat this point, and the wafer was taken out of the reactor forexamination. The wafer surface was visually not specular, and appearedrough. Under microscope examination, the surface was covered with highdensity of pits. This layer was grown under a 3D growth mode.

After growth of the nucleation layer, the growth rate was reduced toapproximately 65 microns per hour by reducing HCl flow to 23 sccm whilekeeping the same NH₃ and growth temperature. After growth ofapproximately 7 minutes under these conditions, the HCl flow wasincreased to 46 sccm, the NH₃ flow was reduced to 1500 sccm and growthtemperature was raised by 15° C. to 983° C. for approximately 1 hour.These two growth conditions were considered as the transitional layerwhere the surface morphology was improved with less pits. Ferrocene wasintroduced in the growth system using a nitrogen carrier gas flow of 200sccm. In some runs, the growth was stopped at this point and wafer wastaken out of the reactor for examination. The wafer surface was visuallyspecular and smooth. A few pits still remained, but the percentage ofthe surface covered by pits was less than 1%. In this step the growthmode was transitioned from the 3D mode of the nucleation to 2D growthmode.

After the growth of the transitional layer, the NH₃ flow was furtherreduced to 900 sccm for additional growth time of 7 hours. The ferroceneflow was maintained during this layer.

After the growth of the bulk single crystalline GaN layer, the growthcondition was changed to grow a mechanically stronger polycrystallineGaN cap. The growth temperature was reduced by 20° C. to 963° C. and theNH₃ flow was increased from 900 sccm to 5000 sccm. The HCl flowincreased to 120 sccm for a nominal growth rate of 330 μm/hr. At thisgrowth condition, the GaN film gradually changed into polycrystallineduring the growth. In this example, the growth time for thepolycrystalline cap was about 4 hours and the nominal thickness of thepolycrystalline GaN material was about 1.3 mm. The total GaN thicknesswas about 2.6 mm.

After completion of the growth sequence, the GaN/sapphire bi-layer wascooled to room temperature at a cooling rate of approximately 6° C. perminute. During the cooling process, the GaN cracked laterally, forming a2″ freestanding GaN article and the sapphire substrate with a thin GaNlayer still attached. Because of the presence of the mechanicallystronger polycrystalline GaN layer, the GaN article did not crackvertically during the laterally cracking separation. The freestanding 2″GaN article was crack-free. The thickness of the freestanding GaNarticle was about 2.1 mm.

The top surface of the freestanding GaN was polycrystalline material andthe bottom surface was the nitrogen face (N-face) of the singlecrystalline GaN. The bottom surface was not flat because it wasgenerated from lateral cracking.

The 2″ freestanding GaN article was processed into 2″ GaN wafer. First,edge grinding was performed to make into within 0.005″ tolerance of the2″ wafer diameter. Because the initial sapphire had a major flat, thegrown GaN article also had a flat. The ground 2″ GaN article is calledwafer blank herein.

The 2″ GaN wafer blank was mounted on a fixture using wax with the frontsurface facing the fixture. The back surface (N-face) was lapped on ametal plate with diamond slurry until a uniform surface finish wasachieved. At this stage, the thickness of the blank was about 1.8 mm.The 2″ GaN wafer blank was removed from the fixture and mounted againusing wax with the back side facing the fixture. The front side waslapped on a metal plate with diamond slurry. About 1.3 mm of the frontmaterial was removed. At this stage, the polycrystalline material wascompletely removed, resulting in a 2″ single crystalline GaN wafer about500 micron thick. Because iron doping was used during the growth, theGaN wafer was semi-insulating with a resistivity greater than 10⁶ohm-cm.

The foregoing examples utilized AlN-coated sapphire as the substrate forthe GaN crystal growth. As previously noted, other substrates, coatedwith another type of Group III nitride such as GaN or more generally(Al, Ga, In)N, may also be used in the present invention and thus areincluded within the scope of the invention. In other HVPE reactorsystems that enable direct nucleation of single crystalline GaN film onbare sapphire substrate, bare sapphire may be used as substrate for thebulk GaN growth according to the present invention. GaN wafers, whichmay be produced according to implementations of the present invention,may also be used as the substrate for the bulk GaN crystal growth. Othersubstrates; such as silicon, silicon carbide, gallium arsenide, lithiumgallate, lithium aluminate, zinc oxide, spinel, may be used as thesubstrate.

The examples of the present invention utilized several specific growthsequences. It should be understood that these specific growth processare meant for illustrative purposes and are not limiting. It should alsobe noted that growth conditions cited in the examples are specific tothe HVPE growth reactor employed in the examples. When employing adifferent reactor design or reactor geometry, it may be desirable toutilize a different condition to achieve similar results. However, thegeneral trends are still similar.

The techniques described in the present disclosure for makingfree-standing GaN substrates from those of the prior art. In one aspect,methods of the present disclosure generate lateral cracks in GaN toseparate GaN from the substrate. In another aspect, methods of thepresent disclosure employ several layers of GaN grown under differentconditions to control the stress in the GaN film and promote theseparation of GaN from the underlying substrate. In another aspect,methods of the present disclosure do not rely on the creation of voidsin the films.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the growth of the bulk GaNcrystal within the scope of the present invention. Thus it is construedthat the present invention covers the variations and modifications ofthis invention provided they come within the scope of the appendedclaims and their equivalent.

What is claimed is:
 1. A semiconductor structure comprising: asubstrate; disposed over the substrate, a single-crystalline layercomprising a first III-nitride semiconductor material; and disposed overthe single-crystalline layer, a polycrystalline layer comprising asecond III-nitride semiconductor material.
 2. The semiconductorstructure of claim 1, wherein the first III-nitride semiconductormaterial and the second III-nitride semiconductor material are the same.3. The semiconductor structure of claim 1, wherein thesingle-crystalline layer comprises a nucleation layer comprising aplurality of pits and, disposed thereover, a bulk layer substantiallyfree of pits.
 4. The semiconductor structure of claim 3, wherein thenucleation layer and the bulk layer each comprise a material selectedfrom the group consisting of GaN, AlN, InN, and ternary and quaternaryalloys and mixtures including one or more thereof.
 5. The semiconductorstructure of claim 3, wherein the single-crystalline layer comprises,disposed between the nucleation layer and the bulk layer, a transitionallayer having a morphology that is less pitted with increasing thickness.6. The semiconductor structure of claim 5, wherein the transitionallayer comprises a material selected from the group consisting of GaN,AlN, InN, and ternary and quaternary alloys and mixtures including oneor more thereof.
 7. The semiconductor structure of claim 1, wherein thesubstrate is selected from the group consisting of sapphire, siliconcarbide, gallium arsenide, zinc oxide, silicon, spinel, lithium gallate,and lithium aluminate.
 8. The semiconductor structure of claim 1,further comprising an epitaxial nitride layer disposed between thesubstrate and the single-crystalline layer.
 9. The semiconductorstructure of claim 8, wherein the epitaxial nitride layer comprises amaterial selected from the group consisting of GaN, AlN, InN, andternary and quaternary alloys and mixtures including one or morethereof.
 10. The semiconductor structure of claim 1, wherein the secondIII-nitride semiconductor material comprises a material selected fromthe group consisting of GaN, AlN, InN, and ternary and quaternary alloysand mixtures including one or more thereof.